The present invention relates to plasma etching chambers. More particularly, the present invention relates to an apparatus for measuring the dc bias voltage of a wafer during plasma processing.
Integrated circuits are typically fabricated on a wafer of semiconductor material such as silicon or gallium arsenide. During the fabrication process, the wafer is subjected to an ordered series of steps, which may include photomasking, material deposition, oxidation, nitridization, ion implantation, diffusion and etching, in order to achieve a final product.
There are two basic types of etches: ion-assisted etches (also called reactive-ion, plasma or dry etches) and solution etches (also called wet etches). Solution etches are invariably isotropic (omnidirectional) in nature, with the etch rate for a single material being relatively constant in all directions. Reactive-ion etches, on the other hand, are largely anisotropic (unidirectional) in nature. Reactive-ion etches are commonly used to create spacers on substantially vertical sidewalls of other layers, to transfer a mask pattern to an underlying layer with little or no undercutting beneath mask segment edges, and to create contact via insulative layers.
A plasma etch system (often referred to as a reactor) is primarily a vacuum chamber in which a glow discharge is utilized to produce a plasma consisting of chemically reactive species (atoms, radicals, and ions) from a relatively inert molecular gas. The gas is selected so as to generate species which react either kinetically or chemically with the material to be etched. Because dielectric layers cannot be etched using a direct-current-induced glow discharge due to charge accumulation on the surface of the dielectric which quickly neutralizes the dc-voltage potential, most reactors are designed as radio-frequency diode systems and typically operate at a frequency of 13.56 MHz, a frequency reserved for industrial, scientific and medical, non-communication use by international agreement. However, plasma etch processes operating between 100 KHz-80 MHz have been used successfully.
FIG. 1 illustrates a conventional method for measuring the dc bias voltage of a wafer in a capacitively coupled plasma etching chamber. A wafer 102 is disposed on an electrostatic chuck 106 inside a vacuum chamber 104. The electrostatic chuck 106 is electrically coupled to an RF generator 110. A grounded upper electrode 108 is disposed inside the vacuum chamber 104 above the wafer 102. During processing, plasma 112 is generated between the upper electrode 108 and the wafer 102. The plasma 112 generates a dc bias voltage above the surface of the wafer 102. The dc bias voltage is conventionally measured outside the vacuum chamber 104 with a measuring device 114, such as a voltage meter, coupled to the electrostatic chuck 106.
The problem with the above approach is that the Radio Frequency (RF) energy is transferred outside the vacuum chamber 104. So the plasma 112 inside the vacuum chamber 104 is disturbed resulting in less accurate measurements of the dc bias voltage. Another disadvantage is that such a system measures the dc bias voltage on the electrostatic chuck 106, and not the wafer 102 itself. There could be a substantial potential difference between the wafer 102 and the electrostatic chuck 106 due to a number of factors including wafer material or coating, process chemistry, RF power level.
A definite need exists for an apparatus for measuring the dc bias voltage in a vacuum chamber. Specifically, a need exists for an apparatus for measuring the dc bias voltage in a capacitively coupled plasma etching chamber. A primary purpose of the present invention is to solve these needs and provide further, related advantages.
An apparatus for measuring the DC bias voltage of a wafer in a chamber comprises an electrical coupling, a fist filter, a second filter. The electrical coupling receives a probe for measuring the DC bias voltage in the chamber. The probe is disposed within the chamber. A first filter, coupled to the electrical coupling, is disposed within the chamber. A second filter, coupled to the first filter, is disposed outside the chamber.